Piezoelectric resonator network



Jan. 26, 1943 w. R. KOCH 2,309,602

PIEZ OELECTRIC RESONATOR NETWORK Filed April 1, 1941 Snventor v 4 attorney Patented Jan. 26, 1943 PIEZOELECTRIC RESONATOR NETWORK Winfield R. Koch. Haddonfield, N. .L, assig'nor to Radio Corporation of America, a corporation of Delaware Application April 1, 1941, Serial No. 386,295

6 Claims. (Cl. 178-44) This invention relates to piezo-electric resonator networks, and has for its primary object to provide a network of the character referred to, having variable band width control without changing the resonant frequencyof the piezoelectric device or devices employed therein.

It is a further object of this invention to provide a signal conveying systemincluding a pair of piezo-electric resonator circuits of the neutralized bridge type, connected with means for varying the band width over a relatively wide range without changing the resonant frequency of piezo-electric resonator devices therein.

In piezo-electric selective networks it is customary to utilize three electrode crystals, each crystal having a main electrode, an input electrode and an output electrode. Small capacities between the three electrodes may provide a non-symmetrical output characteristic and may limit considerably the band width of operation that may be provided. Furthermore, with relatively small crystals, it is difficult to secure the three electrodes thereto and to make suitable connections therewith In order to provide variable band width op eration in connection with narrow band filters using two crystals it is at present considered necessary to change the resonant frequencies of one of the crystals when the band width is changed, or multiple electrode operation of one of the crystals must be provided.

It is an object of this invention, however, to provide variable band width operation in a piezoelectric filter system without changing the resonant frequency of the piezo-electric device or devices therein, and at the same time, to permit two electrode operation of the said device or devices.

In accordance with the invention, two balanced bridge crystal circuits are provided in series in a signa1 conveying system, one piezoelectric device or crystal being provided with a tuned input circuit and the second piezo-electric device or crystal being provided with a tuned output circuit. Band width adjusting means is provided also as coupling means between the two crystals.

With this arrangement it has been found that the selectivity may be varied over a relatively- 'wide range without changing the resonant frequencies of the crystals, by varying the resistance in shunt with the two tuned circuits, either conjointly or separately, thereby to vary the effective resistance in series with the series resonance of the crystals, and by changing the re- -the crystal 8 is indicated at 35 actance of the coupling means between the crystals.

The invention will, however, be further understood from the following description when considered in connection with the accompanying drawing, and its scope is pointed out in the appended claims.

Referring to the drawing, the single figure shows, a piezo-electric resonator network embodying the invention as applied to a signal transmission circuit of an intermediate frequency amplifier.

The network includes an input coupling transformer 5 and an output coupling transformer 6 tuned to a predetermined intermediate frequency to which the two crystals 1, and 8 in the network are also resonant.

The input transformer 5 comprises a primary winding ll) tuned by a shunt capacitor II and a suitable movable magnetic core indicated at I2. The secondary indicated at I3 is tuned by two series connected capacitors I4 and I5 forming a shunt connection for the winding to 'provide a tuned input circuit II for the crystal 1 and .01 the network. The winding is also variably tunable by means of a movable magnetic tuning core indicated at IS. A tap l8 between the capacitors l4 and I5 is provided to ground or chassis I 9.

A variable shunt resistor 2| is provided across the circuit I! in shunt with the winding l3 and the series connected capacitors l4 and I5 as shown. One high potential terminal 22 of the circuit It is connected to the input electrode 23 of the crystal I and the other high potential terminal 24 is connected to the output electrode through an adjustable neutralizing capacitor 26. The output circuit 21 for the crystal 1 with respect to ground or chassis 28 is provided with a shunt coupling reactance such a capacitor 29 and with other suitable coupling reactance means comprising an indu"tance 30 also'connected to ground 3| and to t e circuit 21 through a band changing switch 32.

The output circuit for the network and for and comprises a primary winding 36 for the transformer 6 which is tunable by a movable magnetic core element indicated at 31 and by twoseries connected capacitors 38 and 39 providing a shunt 4 The circuit 35is pro-- 21, while the terminal 48 is connected to the output electrode 49 of the crystal.

. The secondary 50 of the transformer 6 may be substantially untuned and may be connected to an intermediate frequency amplifying stage 5|.

In operation, intermediate frequency signals applied to the tuned primary of the input transformer 5 from supply leads indicated at 52, are transferred through the transformer to the tuned input circuit l1 and thence through the balanced crystal circuit to the coupling circuit 21 across the coupling reactance 29, and the additional coupling reactance 30 when the switch 32 is closed. The signal is then conveyed through the second balanced crystal circuit to the tuned output circuit 35 and through the transformer 6 to the secondary 50 and the amplifier stage 5 I.

The circuits Ill-l l, l I and 35 and the crystal or piezo-eleetric devices I and 8 are responsive to the same intermediate frequency, the circuits l1 and 35 being broadly responsive with the shunt resistors 2| and 42 adjusted for maximum resistance. The capacitors 26 and 46 are adjusted to neutralize the electrode capacities of the crystals. The band width may then be reduced by adjustment of the resistors 2| and 42, a decrease in the resistance causing the selectivity to be increased and the band width to be reduced, for the reason that the effective resistance in series with the series resonance of the crystals is reduced thereby permitting a reduced response at frequencies adjacent to the intermediate frequency selected.

In addition, the band width may be adjusted by changing the coupling reactance at 29-30, in the present example, by opening and closing the switch 32 to remove or add the shunt inductance v the character shown, the selectivity curve or characteristic of a double crystal filter network may be made substantially flat-topped, rounded or double-peaked, as desired, by adjusting the resistance in shunt with the input and the output circuits.

The coupling reactance 29 may be increased by making the capacitor smaller. However, because of the stray capacity of the leads 2! and the electrodes 25 and 41, the minimum capacity obtainable may be too high. By connecting the coil 30 in parallel, the reactance of the coupling circuit, while still being capacitive and reasonably constant over the band desired, is efiectively higher than could otherwise be obtained. The inductance is chosen so as to resonate with the total circuit capacity outside the band of frequencies selected. Several inductance and capacity values may be used to receive different band widths.

I claim as my invention:

1. In a piezo-electric resonator network, the V piezo-electric crystal devices, and means for changing said coupling reactance, thereby to chang the effective band width of said network.

2. In a signal amplifying system, an amplifier coupling network including in combination, a tuned input circuit, a tuned output circuit, a pair of two-electrode piezo-electric crystal devices connected serially in said system between said circuits, means for neutralizing the capacity of said devices in each circuit, selectivity control means providing a variable shunt resistance connection across at least one of said circuits, band width adjusting reactance means, and means for selectively connecting said last named means between said crystal devices to provide a coupling impedance therefor.

3. In a signal conveying system, the combination of a pair of tuned piezo-electric resonator circuits of the neutralized bridge type broadly responsive to a predetermined signal frequency, adjustable reactance means providing a band width controlling coupling element between said circuits, and shunt variable resistance means for said circuits for varying the effective resistance in series with the series resonance of the circuits thereby to vary the selectivity characteristic of the system without changing the piezo-electric resonant frequency of either circuit.

4. A variable band width piezo-electric resonator network comprising in combination, a pair of piezo-electric resonator circuits of the neutralized bridge type broadly responsive to a predetermined frequency, band width adjusting reactance means providing a coupling connection between said circuits, and means for sharpening the selectivity characteristics of said circuits comprising a shunt variable resistor across each circuit.

, 5. In a signal conveying system, the combination of two balanced-bridge crystal circuits, variable inductance means for tuning said circuits to substantially the same signal frequency, a piezoelectric crystal in each of said circuits responsive to said signal frequency, adjustable reactance means for coupling said circuits, and variable resistance means in shunt with each of said circuits for reducing the band width of the response of said circuits as the shunt resistance is reduced, whereby the selectivity characteristic of said circuits is varied without changing the resonant frequencies of said crystals.

6. In a signal conveying system, the combination of two balanced bridge crystal circuits, vari able inductance means for tuning said circuits to substantially the same signal frequency, a piezoelectric crystal in each of said circuits responsive to said signal frequency, band width determining adjustable reactance means coupling said circuits at the high potential terminals thereof serially through said piezo-electric crystals, variable resistance means in shunt with each of said circuits for reducing the effective band width of the response of said circuits as the shunt resistance is reduced, a tuned signal supply circuit coupled to one of said circuits for applying signals thereto, and a signal output circuit coupled to the other of said circuits WINFIELD R. KOCH. 

